Recently, various nonaqueous electrolyte secondary batteries have been developed. Typical nonaqueous electrolyte secondary batteries include lithium ion secondary batteries. Conventionally, carbon materials are mainly used as negative electrode active materials for lithium ion secondary batteries and have been put to practical use. The lithium ion secondary batteries using the carbon materials as the negative electrode active materials are characterized by high electromotive force and high energy density and are used as main power supplies for mobile communication devices and portable electronic devices. Furthermore, lithium-titanium composite oxide materials that can be used as negative electrode active materials have been newly developed and are attracting attention. For example, a lithium ion secondary battery using Li4Ti5O12 as the negative electrode active material has already been put to practical use.
Li4Ti5O12 is a material having a spinel-type crystal structure and can absorb and release Li repeatedly. Accordingly, Li4Ti5O12 can be used as an active material for lithium ion secondary batteries. Li4Ti5O12 absorbs or releases Li at a potential of approximately 1.5 V with respect to the standard redox potential (Li/Li+) of lithium. Therefore, it is considered that when Li4Ti5O12 is used in a lithium ion secondary battery as the negative electrode active material, a highly safe lithium ion secondary battery can be obtained in which a lithium metal tends not to be deposited on the negative electrode even when reaction overpotential is caused by, for example, quick charging. Furthermore, since the lithium ion secondary battery undergoes very small lattice expansion and contraction resulting from charge and discharge and thus has a stable crystal structure, it has good cycle characteristics and is therefore expected to be used for a home electric power storage system, a motor-powered motorcycle, an electric vehicle, a hybrid electric vehicle, etc.
On the other hand, an oxide material having a layered or spinel-type crystal structure is commonly used as a positive electrode active material for lithium ion secondary batteries. Particularly, a material having a layered crystal structure is excellent in terms of high capacity. Typical examples include LiCoO2, LiNi5/6Co1/6O2, and LiNi1/3Mn1/3Co1/3O2.
Therefore, lithium ion secondary batteries are being developed, which are composed of a combination of a positive electrode that contains, as a positive electrode active material, an oxide material having a layered crystal structure and a negative electrode that contains Li4Ti5O12 as a negative electrode active material. For example, Patent Literature 1 proposes a nonaqueous secondary battery in which a lithium titanate compound represented by general formula LiaTi3-aO4 (wherein a indicates a number in the range of 0<a<3) is used as a negative electrode active material and a compound represented by general formula LiCobNi1-bO2 (0≦b≦1) and/or LiAlcCodNi1-c-dO2 (0≦c≦1, 0≦d≦1, 0≦c+d≦1) is used as a positive electrode active material.
Generally, irreversible capacity during the initial charge and discharge of the negative electrode containing Li4Ti5O12 used as the negative electrode active material is smaller than that of the positive electrode containing an oxide material having a layered crystal structure used as the positive electrode active material. Accordingly, the single electrode potential of the positive electrode changes before the single electrode potential of the negative electrode changes at the end of discharge, and thereby the battery voltage reaches discharge cut-off voltage (hereinafter, this is referred to as “positive electrode limitation”. Conversely, the single electrode potential of the negative electrode changes before the single electrode potential of the positive electrode changes and thereby the battery reaches the cut-off voltage, which is referred to as “negative electrode limitation”. Furthermore, both the single electrode potential of the positive electrode and the single electrode potential of the negative electrode change and thereby the battery reaches the cut-off voltage, which is referred to as “positive-negative electrode limitation.”). Therefore, in the case of the battery of Patent Literature 1, irreversible capacity during the initial charge and discharge of the positive electrode is larger than that during the initial charge and discharge of the negative electrode, which results in the positive electrode limitation.
Generally, in the case where the positive electrode contains, as the positive electrode active material, an oxide material having a layered crystal structure, the repetition of insertion and desorption of lithium in a discharge end region where the single electrode potential changes considerably results in a bigger change in crystal structure, which results in a higher cycle degradation rate of the single electrode. Li4Ti5O12 undergoes a very small change in crystal structure caused by insertion and desorption of lithium. Accordingly, the negative electrode containing Li4Ti5O12 has a very low cycle degradation rate of the single electrode even when insertion and desorption of lithium are repeated in the range where the single electrode potential changes considerably. As described above, the conventional battery proposed in Patent Literature 1 is of a positive electrode limitation type. Therefore, in the conventional battery, the potential of the positive electrode changes considerably at the end of discharge, which results in a high cycle degradation rate of the positive electrode as a single electrode, and thus the cycle degradation of the battery becomes pronounced.
Consequently, the addition of a metal oxide such as an oxide containing manganese to a negative electrode is proposed as a technique for suppressing the cycle degradation of the battery caused by the reason described above (Patent Literature 2).